coresight: of: Use of_cpu_node_to_id helper

Reuse the new generic helper, of_cpu_node_to_id() to map a
given CPU phandle to a logical CPU number.

Acked-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Tested-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>